1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of developing units and image forming method.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus performs a repeated cycle of operations involving charging, latent image formation by lithography, development, transfer, discharging, and cleaning. Also, a color electrophotographic image forming apparatus is provided with a developing unit for each color thereby to perform the above-described cycle of operations. Specifically, the color electrophotographic image forming apparatus having plural developing units performs processing which involves forming an image for a single color by the developing unit, transferring the image (or a toner image) to a transfer body, and conveying the transfer body thereby to convey the toner image to the next developing unit. Here, among the plural developing units, the developing unit which transfers the toner image first is defined as an upstream side of a printing unit, and the developing unit which transfers the toner image thereafter is defined as a downstream side thereof.
A discharging unit to eliminate electric charges remaining on a photoreceptor drum has recently been omitted for the purposes of cost reduction. In a case where the discharging unit is omitted, however, a charged image remaining after the transfer of the image affects the following photoreceptor drum on the downstream side and hence affects a printed result on the downstream side. For example, the toner image formed on a transfer belt by the developing unit on the upstream side may also act as the charged image containing electric charges. The toner image containing the electric charges applies the electric charges to the photoreceptor drum on the downstream side which contacts the transfer belt. The photoreceptor drum on the downstream side is not subjected to the discharging by the discharging unit, and therefore causes a phenomenon called a ghost due to the influence of the toner image containing the electric charges on the upstream side.
There is no approach suitable for suppression of the above-described ghost phenomenon. For example, one possible approach to suppress the ghost phenomenon is to increase a charging voltage at which a charging unit charges the developing unit. The reason is that an increase in the charging voltage to the photoreceptor drum leads to a reduction in relative strength of an electric potential reversely transferred from the transfer belt to the photoreceptor drum. With the charging voltage remaining increased, however, there also arises a problem such as excessive consumption of the toner or scattering of the toner onto an unintended portion. To address this problem, although exposure is performed only on a rendered area for typical image formation, feeble light exposure may be performed also on a non-rendered area other than the rendered area thereby to reduce the charging voltage to its allowable range. Although Japanese Patent Laid-Open Nos. H09-169136(1997) and 2003-312050 disclose a technology of performing exposure on the non-rendered area, the technology is not for the purpose of suppressing the ghost phenomenon and thus gives no consideration to a problem resulting from the suppression of the ghost phenomenon.